1. Technical Field
The present invention relates to gas sealing structures for solid polymer electrolyte fuel cells.
2. Related Art
In solid polymer electrolyte fuel cells, a separator is layered on both sides of a plate-shaped membrane electrode assembly to form a unit of the layered structure, and the plural units are layered to form a fuel cell stack. The membrane electrode assembly is a layered structure, in which a polymerized electrolytic membrane is held by a positive catalytic electrode (cathode electrode plate) and a negative catalytic electrode (anode electrode plate), and a gas diffusion layer is layered on the outer surface of each catalytic electrode. The separator is made from a material having electron transmitting characteristics, and has plural grooved gas passages in which a fuel gas such as hydrogen gas, an oxidizing gas such as oxygen or air, and a coolant flow individually. The separator is layered on the membrane electrode assembly such that linear protrusions between the gas passages are contacted with the gas diffusion layer.
According to the fuel cell, a fuel gas is provided to the gas passage of the separator at the negative electrode side, and an oxidizing gas is provided to the gas passage of the separator at the positive electrode side, whereby electricity is generated by electrochemical reaction. During the operation of the fuel cell, the gas diffusion layers transmit the electrons generated by the electrochemical reaction between the catalytic electrode layers and the separators, and diffuse the fuel gas and the oxidizing gas. The catalytic electrode layer in the negative electrode side results in a chemical reaction for the fuel gas so as to generate protons and electrons. The catalytic electrode layer in the positive electrode side generates water from oxygen, the proton, and the electron, and the polymerized electrolytic membrane facilitates ionic migration for the proton, whereby electrical power is provided via the positive and negative catalytic electrode layer.
In the above-described fuel cell, the fuel gas, the oxidizing gas, and the coolant must be flowed in individual gas passages, so that the gas passages are separated from each other by a seal. The sealing portion varies according to the structure of the fuel cell stack. For example, a seal is provided around a communicating opening of the gas passages penetrating the fuel cell stack, around the membrane electrode assembly, around a coolant passage provided on the outer surface of the separator, and around the circumference of the outer surface of the separator.
According to conventional sealing technology, in general, an elastic material made from an organic rubber of the fluorine type, silicone type, ethylene propylene type, or the like, is formed into a shape of a sheet or an O-ring, and is mounted to a sealing portion. The sealing member seals the sealing portion by a reaction force generated by being compressed in a stacked condition. As other sealing structures, a seal in which an inorganic material formed of carbon or ceramic is compressed, and a mechanical seal using caulking, adhering, and the like have been provided.
Fuel cells are often carried or installed in automobiles for use. In these cases, the cells are stringently required to be small and thin. In particular, it has been desired how separators are realized in a structure as thin as possible. Materials of a type, such as baked carbon, which is easily broken by bending, and of a type, such as expanded carbon and metals, which is flexible to a certain extent in bending are known. When the separator made from the materials of the former type is used in assembling a fuel cell stack by stacking plural membrane electrode assemblies, the position of the sealing portion varies due to the variation in the thickness of the membrane electrode assembly and the variation of the size of the separator. As a result, the stresses in the sealing portions differ from each other, and breakage in the separator readily occurs. Therefore, a liquid seal is coated on the separator, and it is cured after stacking the membrane electrode assemblies. In contrast, the separator made from the materials of the later type has flexibility, so that even if a difference in the stresses in the sealing portion occurs, the difference is partially absorbed by the separator, whereby general solid seals may be applied.
In the liquid seal, the steps comprising coating on the separator through curing after stacking the membrane electrode assemblies require complicated control of the viscosity of the liquid seal and degassing therefrom for coating accuracy, and complicated setting of the environmental temperature and the conditions of the coating machine are necessary. In particular, since the viscosity of the liquid seal must be high to maintain the shape of the coating, a long time is required for the coating to obtain accuracy thereof. Furthermore, a large amount of care is required in traveling and stacking the separators in order not to damage the shape of the liquid seal after coating. Thus, the liquid seals have disadvantages in manufacturing efficiency.
In contrast, in the solid seals, the difference between stresses in the sealing portion can be facilitated by reducing the variations of the thickness of the membrane electrode assembly and the size of the separator, so that the desired sealing characteristics can be obtained. However, in this case, the yield of the membrane electrode assembly and the separator decrease, and the manufacturing efficiency decreases.